1. Field of the Invention
The present invention relates generally to semiconductor photodetector arrays, and more particularly to substrate structures and fabrication methods for such arrays.
2. Description Of the Related Art
Detector arrays for sensing electromagnetic radiation include a large number of photodetectors which each simultaneously register incident photons. Detector arrays can be combined with an imaging system that focuses radiation from spatially separate points of a scene onto a focal plane. The detector array is arranged along the focal plane to receive the imaged radiation. Readout electronics transfer the signals generated by the photodetectors to image processing electronics that generate an electronic image of the scene. The signals from spaced photodetectors can also be used to generate positioning information for various uses, e.g., an astronomical star sensor. A spectrometer can be formed by arranging a detector array to receive radiation from a spectral filter. In this use, the detector output signals constitute a spectral image of the light striking the filter.
A variety of compound semiconductor materials are available for detection of electromagnetic radiation across a range of wavelengths, e.g., HgCdTe (mercury-cadmuim-telluride) is a compound semiconductor material that is sensitive in the middle-infrared spectral region. Semiconductors are sensitive to radiation because a portion of photons that strike the semiconductor material are absorbed with the consequent production of mobile carriers, i.e., each absorbed photon produces an electron-hole pair. Photons can also create photocarriers by ionizing impurity atoms and producing single electrons or holes. Photons that are not absorbed pass through the semiconductor material and proceed along a subsequent path.
Semiconductor electronics technology facilitates the concentration of a large number of semiconductor photodetectors, e.g., photoconductors or photodiodes, into a physically small array. Because of their compact nature, such semiconductor arrays are especially sensitive to photon crosstalk between photodetectors; a phenomenon which degrades the electronic image. Photon crosstalk can occur if the subsequent path of the unabsorbed photons of a photodetector intersects any other photodetector in the array. A particularly troublesome crosstalk path occurs in the radiation transparent substrate that is commonly used to carry the array photodetectors.
For example, FIG. 1 shows a prior art detector array 20 which includes an array of photodetectors and a supporting substrate 22. The photoconductor array is exemplified by a pair of spaced photodetectors 23, 24 which are positioned on the front surface 26 of the substrate 22. Generally, the substrate 22 is a grown crystal that has been sawn and ground to its final shape. Typically, the photodetectors are then epitaxially grown on the substrate so that the substrate and the photodetectors form a heterostructure, i.e., a semiconductor structure containing at least two adjacent layers of different chemical composition, but of similar crystalline structure. Fabrication of individual photodetectors may also involve masking and etching operations that are well known in the art.
The photodetectors are appropriately arranged on the substrate's front surface 26 to receive radiation rays, e.g., positioned along a focal plane 27. An exemplary radiation ray 28 is shown to be incident upon one of the photoconductors 24. Some of the photons in the incident radiation ray 28 are absorbed in the photodetector 24 with the consequent generation of electron-hole pairs. If the photodetector 24 is a photoconductor, the generated electron-hole pairs cause a change in its conductivity that is proportional to the photon flux. Alternatively, if the photodetector 28 is a photodiode, the generated electron-hole pairs cause a measurable change in the reverse current of the photodiode that is indicative of the incident photon flux.
Thus, each of the photodetectors of the array can generate a signal in response to the portion of the radiation that is incident upon them and a combination of these signals represents an electronic image of the radiation. If some of the radiation that is initially incident on one of the detectors subsequently travels along a crosstalk path that intersects another of the detectors, the fidelity of the image is proportionately degraded.
FIG. 1 illustrates the substrate crosstalk path between a "sender" photodetector 24 and a "receiver" photodetector 23. Although photoconductors are configured to absorb photons, they typically are only partially opaque to the radiation so that a portion of the incident photons are absorbed and the remainder pass through the photodetector as exemplified by a remainder ray 32 leaving the photodetector 24.
Substrate materials for photodetector arrays are chosen for several attributes which include the following: a) high electrical insulation for isolation of photodetectors, b) similar thermal coefficient of expansion to that of the photodetectors, c) good heat conductivity, and d) crystallographic compatability with the array material. The need for electrical insulation requires a material with a wide bandgap between the conduction and valence bands; as a consequence, semiconductor substrates are generally transparent to the radiation for which the photodetectors are designed.
Because array substrates are typically transparent, the remainder ray 32 can be transmitted through the substrate 20 by reflection between the substrate's back surface 34 and the optically active top surface 26. A particular set of these reflections can cause the ray 32 to follow a crosstalk path 36 that intersects the rear side 38 of the receiver photodetector 23.
Some of the photons that reach the receiver photodetector 23, along the crosstalk path 36, also produce electron-hole pairs and, since the receiver photodetector 23 cannot differentiate photons by their direction of travel, these photons are falsely registered at the image position represented by the photodetector 23. As a result, the electronic image is degraded.
The reflections from the substrate surfaces are mainly due to the difference in the refractive indexes of the substrate 20 and any medium adjoining the substrate. At each of these reflections a portion of the photons are reflected along the path 36 and a portion are lost by passage through the reflecting surface (assuming that the angle of incidence with the surface is greater than the critical angle for total internal reflection). Therefore, the number of photons in the remainder ray 32 diminishes with each lossy reflection. This is graphically indicated in FIG. 1 by the diminishing width of the line that represents the ray 32. In response to this problem of crosstalk, radiation absorptive coatings have typically been applied to either one or both of the back surface 34 and the front surface 26. Although these coatings reduce the number of photons that complete the path 36 by increasing the loss at each reflection, image degradation is still apparent.
The substrate thickness 40 in FIG. 1 must be sufficient to resist breakage due to typically expected functional stresses of the array 20 during its lifetime, e.g., stresses due to handling, vibration and shock. An exemplary prior art detector array intended for detection of infrared radiation employs photoconductors formed of the compound semiconductor HgCdTe (mercury-cadmium-telluride). Suitable substrates for this semiconductor can be formed of the compound semiconductor CdZnTe (cadmium-zinc-telluride) or the compound semiconductor CdTe. The HgCdTe photoconductors are epitaxially grown on the substrate so that the substrate and the photoconductors are crystallographically continuous.
Semiconductors such as HgCdTe are quite fragile and it has generally been found that the substrate thickness 40, for these materials, must be greater than approximately 650 microns to be sufficiently robust to survive the functional stresses unless the substrate 22 is permanently affixed to another structure such as an integrated circuit readout, e.g., as shown in U.S. Pat. No. 5,264,699.